A vehicle, such as an automobile, is provided with systems, including a driving/braking system that transmits a driving force from a propulsive force generating source, such as an engine, to wheels or imparts a braking force thereto, a steering system (steering control system) for controlling the steering control wheels of a vehicle, and a suspension system that elastically supports a vehicle body on wheels, as main mechanisms. Furthermore, in recent years, there has been known a vehicle provided with a variety of electric or hydraulic actuators to actively (positively) control the operations of the actuators according to a traveling condition of the vehicle or an environmental condition or the like as well as passively operating the systems in response to operations (man-caused operations) of a steering wheel (driver's wheel), an accelerator (gas) pedal, a brake pedal and the like performed by a driver, as shown in, for example, Japanese Patent Application Laid-Open No. 2000-41386 (hereinafter referred to as patent document 1).
Patent document 1 proposes a technique in which the feedforward target value of a rear wheel steering angle is determined on the basis of a front wheel steering angle, the feedback target value of the rear wheel steering angle is also determined on the basis of a difference between a reference state amount (a reference yaw rate and a reference lateral acceleration) and an actual state amount (a yaw rate detection value and a lateral acceleration detection value), and then the rear wheel steering angle follows the sum of the target values. In this case, the reference state amount is set on the basis of a front wheel steering angle. Further, the parameters or gains of transfer functions of a feedforward controller, a feedback controller, and a reference state amount determiner are adjusted on the basis of an estimated value of a friction coefficient of a road surface.
However, according to the technique disclosed in the above-mentioned patent document 1, the influences of the characteristics of a road surface reaction force acting on a wheel are not adequately taken into account although the friction characteristics of a road surface are taken into account. More specifically, the values that may be taken by a driving/braking force component and a lateral force component of a road surface reaction force acting on a wheel or a correlation therebetween is subjected to the influence of a side slip of a wheel in addition to the friction characteristic of a road surface. And, the influences of the side slip and the like are not fully considered in the technology disclosed in patent document 1. For this reason, there are cases where a proper road surface reaction force for bringing the actual state amount close to the reference state amount cannot be generated in a wheel even when a rear wheel steering angle is feedback-controlled.
Meanwhile, to control a motion of a vehicle to a desired motion, it is considered desirable to properly control a road surface reaction force as an external force acting on the vehicle. In this case, a moment in the yaw direction generated about the center-of-gravity point of the vehicle can be manipulated by manipulating, for example, a driving/braking force to be applied to each wheel from a road surface. Hence, the driving/braking force to be applied to each wheel from the road surface could be manipulated to control the behaviors of the vehicle such that an actual state amount related to a motion in the yaw direction approximates a reference state amount.
However, in this case, as described above, the driving/braking force component and the lateral force component of a road surface reaction force are subjected to the influences of not only the friction characteristics, such as the coefficient of friction between a wheel and a road surface, but also a side slip of the wheel. Therefore, it is desired to manipulate the driving/braking force to be applied to a wheel while properly considering such characteristics of a road surface reaction force.
The present invention has been made in view of the background described above, and it is an object thereof to provide a vehicle control device capable of properly controlling a motion of an actual vehicle to a desired motion while properly considering the characteristics of a road surface reaction force acting on a wheel from a road surface. Another object is to provide a vehicle control device capable of enhancing the robustness against disturbance factors or changes therein, allowing a vehicle motion to be properly controlled.